Superluminescent light-emitting diode with reverse biased absorber

ABSTRACT

A non-lasing edge-emitting LED (30) suitable for precision reflectometry has an absorber region (52) which is reverse-biased via a contact (58) to absorb light by Stark absorption or Franz-Keldysch effect. During operation, the gain region (50) is forward biased via contact (56) to produce light emission including stimulated emission from an edge of the device, The absorber region is sized to a length L.sub.α &gt;(gL g  -1/21n(1/R 1  R 2 ))/α where g and α are coefficients of gain and absorption and R 1  and R 2  are the front and back facet (60, 64) reflectivities, such that round-trip power loss through the cavity is at least 60 dB. The length L.sub.α  is sufficient to preclude regenerative oscillation of light in the cavity during light emission including stimulated emission. Antireflection measures further reduce end facet reflectivity, limiting signal contributions due to internal reflections to less that -85 dB below the primary output signal. Controlling cavity width reduces sidewall reflections and using step-biased segmented contacts reduces gain/absorber interface reflections.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor lasers andlight-emitting diodes and more particularly to a nonlasingsuperluminescent LED suitable for applications requiring a low internalreflectivity source.

There is a substantial need for fast and bright sources for opticalcommunications and other purposes. At wavelengths of 1.3 and 1.55microns, all sources suffer from Auger recombination, an essentiallynonradiative process which competes with the active radiative mechanism,thus reducing the light output. In a normal light-emitting diode (LED),the radiative mechanism is spontaneous bimolecular radiativerecombination. In a laser, the radiative mechanism is stimulatedemission. Both the Auger recombination and the bimolecular recombinationrates increase with increasing carrier density, but the Auger rateincreases more rapidly. Because Auger recombination is so strong, thecarrier density must be kept low for high efficiency, but then the decayrate is so slow that the LED is not very fast. Alternatively, thecarrier density can be driven higher so that the decay rate is fast, butthen Auger recombination dominates and the LED is very dim.

Stimulated emission is a more rapid process than bimolecularrecombination at the lower carrier densities mentioned above, thuscompeting more effectively with Auger recombination. Quantum well solidstate lasers with fast switching of large optical signals are describedin K. Berthold, et. al., Voltage-controlled Q switching of InGaAs/InPsingle quantum well lasers, Appl. Phys. Lett. 55(19), pp. 1940-42, Nov.6, 1989, and D. R. Dykaar, et. al., Large-signal picosecond response ofInGaAs/InP quantum well lasers with an intracavity loss modulator, Appl.Phys. Lett. 56(17), pp. 1629-31, Apr. 23, 1990. These devices have aquantum well structure having a cavity length L_(c) of 300 micronsdivided into long gain regions (L_(g) =268 micron) and a shortintracavity absorption region (L_(A) =2-20 microns) that can be biasedwith 6 micron separations (total L.sub.α =14-32 microns) . With asuitable combination of forward and reverse biases applied respectivelyto the gain and absorption regions, the laser diode can be made tooperate in either normally-on or normally-off states, and can beactively Q-switched by voltage control to generate short optical pulses.These devices have been adapted to digital optical switching by A. F. J.Levi et. al., Multielectrode quantum well laser for digital switching,Appl. Phys. Lett. 56(12), pp. 1095-97, Mar. 19, 1990.

The output of a laser may suffer, however, from problems of catastrophicdegradation. Importantly, the narrow bandwidth of a laser optical outputis unsuited for many applications such as optical time domain andoptical coherence domain reflectometry (also called precisionreflectometry). For precision reflectometry, it is preferable to have asource that provides a high power output over a broader bandwidth than alaser provides. Coherence domain reflectometry also requires a lowreflectivity light source, preferably having internal reflected signalamplitudes 70 dB or more below the output signal level.

Superluminescent LEDs (SLEDs) utilize stimulated emission as theirprimary radiative mechanism, but do not exceed the threshold foroscillation. They do not form hot spots and do not catastrophicallydegrade. SLEDs made of bulk material tend to lase when the temperatureis lowered, however, confining the operating specifications to animpractically narrow range of temperature. To prevent lasing at longwavelengths, pumping must be further decreased, at the cost of outputpower.

Because of this problem (and perhaps others), researchers haveconcentrated on fabricating SLEDs using low mirror losses to preventregenerative oscillation. Antireflection coatings are difficult tomanufacture, however, and these devices are very susceptible to lasingas a long cavity laser if light is reflected back into them from anexternal surface. Another approach, employed in a commercially-availableLED offered by Laser Diode Inc. of Edison, N. J. (LDI) for use inreflectometry, uses a 178 micron active gain region and a 533 micronpassive absorber, both formed in bulk material. This device has aninternal reflection signal power of about 40 dB below the main outputsignal even with 3% antireflection coatings on both facets and lowinternal reflectance. While the device is usable and better than otheravailable sources, this level of reflections is still much higher thanoptimal for precision reflectometry.

Accordingly, a need remains for faster, broadband, and bright sourcesfor optical communications, reflectometry and other uses.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to obtain a brighter non-lasinglight source with a broader bandwidth than produced by either aconventional, SLED or semiconductor laser.

Another object of the invention is to prevent regenerative oscillationof a superluminescent or edge-emitting LED over a wide temperaturerange.

A further object of the invention is to reduce the criticality ofantireflection coatings, to avoid introducing sources of reflections,and preferably to minimize reflections and their contribution to thedevice output beam.

The invention is a non-lasing edge-emitting LED comprising asemiconductor heterostructure body including a stack of parallel layersforming a PN junction to define an elongate optical cavity. The cavityincludes a gain region of a length L_(g) and an optical absorber regionin series with the gain region. The optical absorber region has anabsorption spectrum which absorbs a spectral portion of lighttransmitted from the gain region. The device includes contact means forbiasing the PN junction, including a gain contact for forward biasingthe gain region so as to produce light emission, including stimulatedemission, at an edge of the stack (the front facet), and an absorbercontact for reverse biasing the absorber to shift the absorptionspectrum to a lower energy, thereby blocking a greater proportion of thespectrum of the light transmitted from the gain region. The opticalabsorber region must have a length L.sub.α sufficient to precluderegenerative oscillation of light in the cavity during light emission.

The absorber region is sized to a length L.sub.α such that the roundtrip gain of light reflected through the cavity from the front backfacets and through the absorber is less than unity during lightemission. In practice, this requires a reverse-biased absorber lengthL.sub.α of at least 50 microns, preferably equal to or exceeding thelength L_(g) of the gain region (e.g., 300 microns). For very lowinternal reflectance (<-70 dB) applications, the absorber length L.sub.αshould preferably be about 1 mm. or longer. Preferably, the invention isimplemented in a quantum well structure having a quantum-confined Starkeffect absorber, and may be referred to as either a superluminescent LED(QWSLED) or quantum well edge-emitting LED (QWEELED).

This device fulfills the need for a fast, bright emitter which does notcatastrophically degrade and in which laser operation at coldtemperatures can be prevented. Quantum well SLEDs can be faster thanconventional LEDs for comparable output intensities. Although not asbright as lasers, QWSLEDs do not suffer from catastrophic degradationand thus should be more reliable than lasers.

Using a long optical Stark absorber in series with the quantum wellstructure prevents lasing at all practical temperatures even when thegain region is biased to provide a high power optical output. It isadvantageous compared to reducing reflections with antireflectioncoatings alone because long absorbers are easier to fabricate thanantireflection coatings and because the long-absorber quantum well SLEDstructure is less susceptible to external feedback. Anti-reflectionmeasures can be used in addition, however, and are used in a preferredembodiment designed for use in precision reflectometry applications.Such measures are less critical and more effective than in conventionalSLEDs.

In quantum well edge-emitting LED devices according to the invention, itis easier to prevent lasing at low temperatures and still maintain astrong superluminescent output than in bulk SLEDs. If lasing is notprevented at low temperatures simply by using an unbiased quantum wellabsorber, as may occur if bandgap renormalization (the decrease inbandgap at high carrier densities) in the gain region forces the bandedge to longer wavelengths, a reverse bias can be applied to contacts onthe absorber to move the absorption spectrum out to longer wavelengthsvia the Stark effect. The absorption curve shifts to longer wavelengthswith an electric field applied normal to the plane of the quantum wellsmore nicely than the absorption curve shifts with applied electric fieldin bulk materials. Thus, a QWSLED utilizing the Stark effect in theabsorber attenunates reflections better than a similar bulk SLED.Moreover, laser action can be prevented without modifying doping of theactive gain region, which would affect its refractive index and increasereflectivity within the device.

The foregoing and other objects, features and advantages of theinvention will become more readily apparent from the following detaileddescription of a preferred embodiment of the invention which proceedswith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of absorption and gain coefficients α and gcalculated for GaAs at different current densities.

FIG. 1A is a diagram of the gain and absorption curves comparing theeffect of optical Stark absorption under no-bias and reverse-biasconditions.

FIG. 1B is a diagram of intensities of primary and reflected lightsignals in a coherence domain reflectometry system.

FIG. 2 is a partial sectional and perspective view of the active devicestructures of a quantum well superluminescent LED (QWSLED) with opticalStark absorber in accordance with the invention.

FIG. 2A is a top plan view of a preferred, low-internal reflectanceembodiment of the device of FIG. 2.

FIG. 2B is a more detailed transverse sectional view of the device ofFIG. 2.

FIG. 3 is a diagram of the linear absorption spectra of room-temperatureGaAs and multiple quantum well (MQW) devices.

FIG. 4 is a diagram of a time-integrated photoluminescence spectrum of amultiple quantum well device.

FIG. 5 is a lengthwise sectional view of an operative example of adevice in accordance with the invention.

FIGS. 6-8 are diagrams of the geometry of anti-reflection measures in anabsorber as used in the present invention.

FIGS. 9, 9A, 9B and 9C are sections of alternative embodiments ofanti-reflection absorbers in the present invention.

FIGS. 10, 11 and 12 are top plan views of alternative embodiments ofbiasing structures in the present invention.

FIG. 13 is a source spectrum of a prior art LED source used in precisionreflectometry.

FIGS. 14A and 14B are precision reflectometer spectra showing theadvantages of the device of the present invention.

FIG. 15 is a plot back facet reflectivity against absorber reverse biasvoltage in the device of the present invention.

FIG. 16 is a spectrum like FIGS. 14A and 14B showing the reduction ofinternally reflected signal components from increasing device stripewidth and adding antireflection coating to the front facet.

DETAILED DESCRIPTION Analysis of Problems in the Prior Art

The preferred embodiment of the invention is a quantum welledge-emitting or superluminescent LED (QWEELED or QWSLED) with aquantum-confined optical Stark absorber arranged to precluderegenerative oscillation and resultant laser action, particularly asoccurs at low temperatures in conventional superluminescent LEDs. Astemperature of a SLED decreases, the bandgap energy of the semiconductorincreases. Superposed on this shift, the gain spectrum becomes muchnarrower and sharper and moves towards longer wavelengths, as the Fermifunction redistributes electrons closer to the conduction band edge.Apparently, even if the pumping level is decreased so that the gain peakmagnitude is below the highest absorption magnitude, the gain may stillexceed the losses at long wavelengths and a conventional SLED will lase.

This problem is illustrated in part by FIG. 1, which is reproduced inpart from H. D. Casey, Jr. and B. Panish, Heterostructure Lasers, PartA: Fundamental Principles, New York, Academic Press, 1978, p. 172,quoted from F. Stern, J. Appl. Phys., 47, 1976, p. 5382. The problem isshown further by FIG. 1A, which shows a gain curve 12 for a SLED and anabsorption curve 14 for an unbiased absorber, with an area of overlap 16between the tails of energy curves 12, 14 which produces net gain, whichcan lead to regenerative oscillation. In contrast, a heavily-pumped SLEDwith a long, reverse-biased absorber in accordance with the inventionhas a gain curve 18 shifted by band gap renormalization and anabsorption curve 20 that is shifted under reverse bias so that the gaincurve is contained within the absorption curve. This means there isessentially no wavelength at which the gain exceeds the absorption tocause regenerative oscillation. The peak absorption α_(p) ×L.sub.α ispreferably at least 60 dB over the peak gain g_(p) ×L_(g) to suppresslasing. The quantum well structure of the preferred embodiment furtherdescribed below easily exceeds this objective.

The invention further serves to minimize reflected components in theoutput signal which, in conventional SLED devices, are an obstacle totheir use in reflectometry or require critical antireflection measures.This problem is illustrated in FIG. 1B which shows reflectivity as afunction of distance. The light signal 22 emitted by an LED transmittedthrough an optical system, such as an optical fiber, will be reflectedback as an attenuated signal 24 from any reflective anomalies in thesystem. If the emitted light signal includes an internally reflectedsignal component 26, both internally-reflected signal 26 and externallyreflected signal 24 will be superposed such that user may confuse theinternally reflected signal 26 with an external reflection from a defectat some distance.

Ideally, a source for coherence domain reflectometry has a substantiallyuniform spectral output with no internally reflected signals 26. If thesource has considerable internal reflectivity, e.g., -10 dB, it willproduce a nonuniform output spectrum (as a function of wavelength),caused by increased stimulated emission at the oscillation modes of thecavity. If the internal reflectivity is substantially lower, e.g.--40dB, this nonuniformity will not be observed in the optical spectrum.However, this spectral nonuniformity will produce a confusingreflectrometry signal as shown in FIG. 13 for the LDI source. Thelargest peak 13 is the reflection response of the main EELED signal froma mirror. The peak 15 results from an internal reflection from thegain/absorber interface. And peak 17 results from a reflection from theback facet.

Thus, it is important to suppress internally-generated reflections in alight source for reflectometry. This is done in the invention by using along, reverse-biased absorber, enhanced by the antireflection measuresand quantum well active region discussed below.

Description of construction and operation of the invention

FIGS. 2, 2A, 2B and 5 illustrate a preferred embodiment of a on-lasingedge-emitting LED according to the invention in the form of a quantumwell edge-emitting LED (QWEELED) 30. The physical device structurecomprises a semiconductor heterostructure body including a stack ofparallel layers forming an active region 32 with a PN junction to definean elongate optical cavity. The active region 32 consists of a multiplequantum well (MQW) structure in the form of a stack of alternatinglayers of GaInAs or GaInAsP quantum wells 34 (approximately 40 Angstromswide for 1.3 micron emission) and GaInAsP barriers 36 (e.g., 100-200Angstroms) between the wells. The doping should be p-n or p-i-n,arranged for good carrier injection into the active region. The stack isformed by a conventional fabrication process as an elongate stripe whichcan have a conventional stripe with of about 2 microns but preferably atleast 7 microns, e.g., 8 microns.

Above and below the active region of the long wavelength QWSLED arelayers 40, 42 of InP, which has a higher bandgap than GaInAs and thusforms a good waveguide cladding while confining carriers vertically inthe active region. The device is grown on an InP substrate 44, typicallywith a buffer layer 46, both doped n-type, and with lateralsemi-insulating isolation layers 48, for example of InP:Fe, as shown inFIG. 2A.

The cavity includes a gain region 50 of a length L_(g) and an opticalabsorber region 52 in series with the gain region, with contacts formedexternally to the p and n outer layers for biasing the PN junction,requiring the deposition of contacting layers 56, 58. Rather than twocontinuous contacts across the device, however, one of the contacts mustbe split in at least one location to allow a forward bias to be appliedto the gain region while a reverse bias is applied to the absorptionregion for the Stark effect. Any number of lateral structures can beused as long as the gain path is forced to be continuous with anabsorption path.

Referring to FIG. 2B, the lower or n-type contact 54 is formed on thebackside of the substrate 44. Two metal on p-type GaInAs contacts areformed over the active region, each extending along a portion of thelength of the cavity. The first contact is a gain contact 56 alignedover the gain region 50 for forward biasing the gain region. The gainregion and contact are sized to a length L_(g) and biased in operationto produce light emission including stimulated emission from an edge ofthe stack at the front facet 60 of the device. The output signal can betaken at the gain end of the device for the highest level. This facetmay be antireflection (AR) coated to some degree if desired, consistentwith concerns about reproducibility of the AR coating and externalfeedback.

The second contact is an absorber contact 58 for reverse biasing theabsorber region to shift the absorption spectrum therein to a lowerenergy. The absorber contact is spaced lengthwise from the gain regionby a gap 62 and is sized to a length preferably at least as great as thegain contact to form an absorber region of a total length L.sub.α suchthat the round trip gain of light reflected through the cavity from theback facet 64 of the absorber is less than unity, during light emissionincluding stimulated emission from the front facet. The optical absorberregion should have a length L.sub.α sufficient to preclude regenerativeoscillation of light in the cavity during light emission, and preferablybe long enough to assure that output reflections remain at least 70 dBbelow the output signal.

In an operative example of device 30 shown in FIG. 5, the gain regionhas a length L_(g) of 300 microns, the spacing between the gain andabsorber contacts is 100 microns, the absorber contact is 900 microns inlength and the absorber region has a total length L.sub.α of 1000. Inpractice, this requires a reverse-biased absorber length L.sub.α of atleast 50 microns, preferably equal to or exceeding the length L_(g) ofthe gain region (e.g., 300 microns). For very low internal reflectance(<-70 dB) applications, the absorber length L.sub.α should be about 1mm. or longer. With a very long absorber region, e.g., at least 1 mm.,reverse biasing of the absorber is not essential to precluderegenerative oscillation, but it will reduce the magnitude of internalreflections.

Device 30 is shown as an integrated device, with both the gain andabsorption regions on one wafer, but the device does not have to beintegrated. This device can be fabricated in a number of III-V materialssystems; for example, GaAs/AlGaAs at <1 um and GaInAs/GaInAsP/InP orGaInAsP/GaInAsP/InP at >1 um. Lattice-matched systems are preferred forreliability and low nonradiative recombination, but non-lattice matchedsystems, such as GaInAs/GaAs/AlGaAs at about 1 um, may be used also.Other materials systems, for example, GaAlAsSb, can be used. Because ourgreatest interest is optical communications at 1.3 um and because ofexperience with lattice matched Ga(0.5)In(0.5)As/InP quantum wells atthis wavelength, GaInAs/GaInAsP/InP is used as the example. In practice,higher quality 1.3 um emitting quantum wells may be grown inGaInAsP/GaInAsP/InP.

The use of an MQW active region rather than a single quantum well (SQW)or a few QWs is appropriate for long wavelength emitters because of theneed to keep the carrier density low to avoid Auger recombination. Forshorter wavelengths, such as in the GaAs/AlGaAs system, where Augerrecombination is not a problem, a SQW or few-QW structure may be bestbecause it favors radiative recombination over linear nonradiativemechanisms.

In operation it is necessary to stay below lasing threshold, which isgiven by the expression below, at all wavelengths

    R.sub.1 R.sub.2 exp[2(gL.sub.g-αL.sub.α)]=1

where R₁ and R₂ are the facet power reflectivities, g is the gain percm, L_(g) is the length of the gain region in cm, α is the absorptionper cm, and L.sub.α is the length of the absorption region per cm. Thisintensity condition for oscillation can be rewritten as

    gL.sub.g =αL.sub.α +1/21n(1/R.sub.1 R.sub.2)

after taking the natural logarithm. The device of FIG. 5 readily meetsthis condition at all wavelengths.

In general, the absorption spectrum in these semiconductors lookssimilar to the density of states in the conduction band. However,absorption may take place below the bandgap if the doping level is highenough to cause the formation of bandtails. The absorption curve for aGaAs/AlGaAs MQW structure with relatively low doping is reproduced inFIG. 3 from D. A. B. Miller, D. S. Chemla, D. J. Eilenberger, P. W.Smith, A. C. Gossard and W. T. Tsang, Large room-temperature opticalnonlinearity in GaAS/Ga(1-x)A1(x)As multiple quantum well structures,Appl. Phys. Lett., 41(8), Oct. 15, 1982. The two sharp peaks are due toabsorption into excitons, which are bound electron-hole pairs within thesemiconductor. The photoluminescence of a similar structure, also grownby A. C. Gossard, is shown in FIG. 4, reproduced from J. E. Fouquet andA. E. Siegman, Room-temperature photoluminescence times in aGaAs/Al(x)Ga(1-x)As molecular beam epitaxy multiple quantum wellstructure, Appl. Phys. Lett., 46(3), Feb. 1, 1985. The emission spectrumis sharper on the lower energy side of the peak, representing theconstant density of states above the bandedge. The gradual decrease onthe high energy side is the result of the Fermi function filling of thequantum well. Emission may be present at longer wavelengths if thestructure is heavily doped due to bandtailing or if the structure isheavily pumped due to bandgap renormalization.

If the structure lases at long wavelengths at room temperature, thequantum wells in the gain region may be disordered slightly to shiftgain to shorter wavelengths, thereby preventing lasing. This effect onthe gain spectrum is permanent. The absorption spectrum can also betemporarily shifted to longer wavelengths by applying an electric fieldperpendicular to the wells, as described in, for example, I. Bar-Joseph,C. Klingshirn, D. A. B. Miller, D. S. Chemla, U. Koren and B. I. Miller,Quantum-confined Stark effect in InGaAs/InP quantum wells grown byorganometallic vapor phase epitaxy, Appl. Phys. Lett., 50(15), Apr. 13,1987. This effect, called the Stark effect, can be used to preventlasing at low temperatures in a device which does not normally lase atroom temperature. When the temperature again rises, the electric fieldcan be removed and the SLED can function as it originally did at roomtemperature.

In an operative example of the device in FIG. 5, the 300 micron gainregion was forward biased to a current of 75 mA, while the 900 micronabsorber contact was biased from -2 V. to -4 V. The reverse-biasedquantum confined Stark absorber alone reduced the level of back facetreflections of the EELED to -75 to -85 dBc. FIGS. 14A and 14B showspectra of this device when the absorber region is reverse biased atzero and at -4 V. The main signal peak is indicated by reference numeral80. Proceeding laterally from signal peak 80 are lower but stillnoticeable V-shaped signals 82 culminating in peaks 84. Peaks 88 in FIG.14A are due to the back facet internal reflection, which is onlyminimally attenuated by the absorber at zero volts reverse-bias. In FIG.14B, when the absorption region is reverse-biased at -4 volts, signalpeaks 88 virtually disappear into the detector noise. When the receivergain is increased and bandwidth is decreased to heighten sensitivityover detector noise, it can be seen in FIG. 14B that peaks 88 are 75 to85 dB below primary signal 80 when the absorption region is reversebiased at -4 volts.

FIG. 15 shows how back facet reflectivity decreases as the magnitude ofreverse bias on the absorber is increased. Increasing the magnitude ofreverse bias also increases absorption more strongly at longerwavelengths. A shorter absorber region, however, does not work as well.For example, operating the foregoing type device backwards, applying abias of -2 to -3 volts to the short contact and a forward bias currentof 75 mA to the long contact yields reflectivity signals between -60 and-65 dB. This is better than the LDI device, even though the absorberhere is 300 micron vs. 533 microns for the LDI device and the devicehere lacks AR coatings. This advantage is due to use of both a quantumwell structure as the active region and reverse biasing the absorber.Adding a conventional three layer broadband anti-reflection (AR) coatingto the front (not back) facet 60 reduced all reflection signal featuresby 25 to 30 dB. The resulting reflection from the back facet is so lowthat it can no longer be measured when a reverse bias is applied to theStark absorber.

While the back facet reflection was the most pressing problem with thestandard EELED outputs because of its narrow width and large signalstrength, EELEDs such as that measured in FIGS. 14A and 14B also produceother reflection signal features. The broad reflection signal 82 fromthe gain region probably arises from scattering off of the edges of themesa which is formed during processing. Signal peaks 84 are due to aninternal reflection from the gain/absorber interface. Adding the ARcoating and using wider (about 8 micron) stripes enables reduction ofall reflected signal features (82, 84, 88) to less than -85 dBc as shownin FIG. 16. The shot noise limit of the detection system is at about -85dBc. Thus there is no point to reducing the reflections any fartherbecause it is impossible to measure any signal below -85 dBc.

The device demonstrated in FIGS. 14-16 used a multiple quantum well(MQW) structure for the active region, which proves to be a particularlyeffective absorber. The invention can also be implemented using a bulksemiconductor active region. This device should put out more power butprobably not absorb as well, so back facet reflections may be larger.The Franz-Keldysch effect moves the absorption edge of the bulk materialto longer wavelengths, but it is a weaker effect than the Stark effectin quantum wells. Other measures can be taken to reduce internalreflections, however, besides using an AR coating on the back facet.

FIGS. 6-8 are diagrams of the geometry of anti-reflection measures in anabsorber as used in the present invention. FIG. 6 shows the general caseof partial and total internal reflection of incident light at adielectric interface. From this relationship, the back facet of theabsorber can be inclined to reduce the amount of light reflected backtoward the gain region. It can be inclined in a vertical direction asshown in FIG. 7, e.g., by wet etching which produces a θ_(n) =54.70.This angle causes the incident light to be totally internally reflectedat an angle off the axis of the cavity so that it is not conducted backinto the waveguide except for some scatter.

Alternatively, the back facet is slightly inclined laterally as shown inFIG. 8 to transmit the majority of the light out through the back facetand reflect the remainder internally but off axis so as not to bewaveguided, e.g., θ at least 8°. FIGS. 9, 9A and 9B are sections ofalternative embodiments. FIG. 9 shows a tilted stripe which an angleboth end facets. FIG. 9A shows a truncated stripe with a tilted endetched into the waveguide stack. FIG. 9B shows a curved absorber. FIG.9C shows an absorber 79 with a straight channel terminating in an etchedback facet 72 as in FIG. 9A but broadened asymmetrically to form a beamdump 81.

Another source of internal reflections is the interface between theforward biased gain region and the reverse biased absorber region, dueto variations in carrier density under different bias conditions. Thiscan be alleviated by grading the bias between the two regions. FIGS. 10,11 and 12 are top plan views of alternative embodiments of biasingstructures. In FIG. 10, there are a series of step contacts 57A, 57B,57C which can be biased in steps to stretch the distance over which thediode turns on and thereby extend the distance over which the carrierdensity changes, thus reducing the peak amplitude of the reflections.This can be implemented using a simple voltage divider to apply steppedvoltages to the step contacts and the absorber contact. Alternatively,the gain and absorber contacts can be separated by a diagonal gap 62A,as with contacts 56A, 58A shown in FIG. 11, which distributes theturn-on equipotential at an angle to the waveguide axis so that anyinternal reflections are off-axis and will not be waveguided. Theapproaches of FIGS. 10 and 11 are combined in FIG. 12, which has asingle parallelogram-shaped step-down contact 59.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventioncan be modified in arrangement and detail without departing from suchprinciples.

I claim all modifications and variation coming within the spirit andscope of the following claims:
 1. A non-lasing edge-emitting LEDcomprising:a semiconductor heterostructure body including a stack ofparallel layers forming a vertically-confining waveguide and a PNjunction to define an elongate optical cavity containing an activeregion; the cavity including a gain region of a length L_(g) and anoptical absorber region in series with the gain region, the opticalabsorber region having an absorption spectrum; and contact means forbiasing the PN junction, including a gain contact for forward biasingthe gain region so as to produce light emission including stimulatedemission from the active region along an edge of the stack, and anabsorber contact for reverse biasing the optical absorber region toreduce the optical absorber region band edge photon energy below thezero bias band photon energy; the optical absorber region, under reversebias, having a length L.sub.α sufficient to preclude regenerativeoscillation of light in the cavity during light emission.
 2. Anon-lasing edge-emitting LED according to claim 1 in which the absorberregion is sized to a length L.sub.α of at least 50 microns.
 3. Anon-lasing edge-emitting LED according to claim 1 in which the absorberregion is sized to a length L.sub.α equal to or exceeding the lengthL_(g) of the gain region.
 4. A non-lasing edge-emitting LED according toclaim 1 in which the absorber region is sized to a length L.sub.α ofthree times or more the length L_(g) of the gain region.
 5. A non-lasingedge-emitting LED according to claim 4 in which the gain region is sizedto a length L_(g) of about 300 microns.
 6. A non-lasing edge-emittingLED according to claim 1 in which the absorber region is sized to alength

    L.sub.α >(gL.sub.g -1/21n(1/R.sub.1 R.sub.2))/α,

where g and α are coefficients of gain and absorption and R₁ and R₂ arereflectivities of front and back facets of the cavity, such thatround-trip power loss through the cavity is at least 70 dB.
 7. Anon-lasing edge-emitting LED according to claim 1 in which the bodycomprises a strongly-laterally index guided heterostructure laterallyisolated by a nonconducting material to define an index-guided cavityfor stimulated emission in the gain region and Stark absorption in theabsorber region.
 8. A non-lasing edge-emitting LED according to claim 1in which the contact means includes an intermediate contact spacedbetween the gain and absorber contacts for biasing to an intermediatebias relative to the gain bias.
 9. A non-lasing edge-emitting LEDaccording to claim 1 in which the stack is sized to a stripe width ofabout 8 microns.
 10. A non-lasing edge-emitting LED according to claim 1in which the active region comprises a quantum well structure.
 11. Anon-lasing edge-emitting LED according to claim 1 in which the opticalabsorber region is sized to a length L.sub.α sufficient, under reversebias, to limit round trip reflection of light in the cavity to less than-60 dB.
 12. A non-lasing edge-emitting LED according to claim 1 in whichthe contact means includes means for controlling a change in refractiveindex of the cavity at an interface between the gain and absorberregions to suppress internal reflections from the interface.
 13. Anon-lasing edge-emitting LED according to claim 1 in which the stackincludes a back facet defining one end of the cavity in the absorberregion, the back facet being angled so as to suppress internalreflection of light in the waveguide.